1. Field of the Invention
The present invention relates to a codebook preliminary selection device, a codebook preliminary selection method and a storage medium storing a codebook preliminary selection program, for use in speech signal coding.
2. Description of the Related Art
In order to satisfy the requirements for increased capacity and reduced power consumption for mobile communication system, such as automobile telephone systems, standardization for low bit rate speech signal coding has been developed. The PSI-CELP (Pitch Synchronous Innovation-Code Excited Linear Prediction) is one of such well known standards. In PSI-CELP, 3.45 kbit/s is used for speech coding. In the speech coding, various processes, such as noise canceling, LSP quantization, power quantization, adaptive/fixed codebook searching and stochastic codebook searching, are carried out. This technique is introduced, for example, in the following literature:
Pitch Synchronous Innovation CELP (PSI-CELP) PA1 --PDC half-rate speech CODEC-- PA1 Tomoyuki Ohya and collaborators PA1 The Institute of Electronics, Information PA1 and Communication Engineers PA1 Technical Report of IEICE, RCS93-78, 1993-11
In the PSI-CELP, vocal tract information and excitation source information are encoded separately and then multiplexed to form a total code for a speech signal. Specifically, for encoding an input speech signal, LPC (Linear Predictive Coding) is first performed to obtain LPC parameters (coefficients) which represent the vocal tract information. The excitation source information is encoded, for example, through vector quantization of LPC predictive errors and stored in codebooks, such as an adaptive codebook, in the form of excitation code vectors with corresponding indices. When encoding the excitation source information of the input speech signal, the adaptive codebook is searched in the following manner: Specifically, the code vectors stored in the adaptive codebook are selected individually and inputted into an LPC synthesis filter so as to obtain synthetic speech signals. The synthetic speech signals are compared with the input speech signal individually so that such a code vector that minimizes a perceptual error between the synthetic speech signal and the input speech signal is finally selected as optimally representing the excitation source information of the input speech signal.
In the foregoing technique, however, there has been the following problem:
Specifically, the foregoing codebook searching requires repetition of calculations using the input speech signal and a large number of excitation code vectors, thus resulting in enormous amounts of calculations. In view of this, the foregoing literature introduces a technique of preliminary selection so as to reduce the amount of calculations. In the preliminary selection technique, simple evaluation values for all the candidates (code vectors) stored in the codebook are derived and then high-ranking X candidates are selected based on these simple evaluation values. However, for selecting the high-ranking X candidates, calculations of comparison of approximately (all the candidates.times.X) times are required. This calculation amount is not negligible and impedes load reduction and increased speed of a processing unit.